1. Field of the Invention
The present invention relates to the technique for extracting target signals from time-series signals, and more specifically, to signal extraction using the independent component analysis with reference.
2. Description of the Related Art
A conventional fetal phonocardiogram basically monitors faint pulses emanating from a fetal heart using a microphone placed on a maternal body surface, and therefore the measurement is limited to be conducted in a resting environment with lower ambient noises for only a short time period, so that the technique hardly gains the attention these days and is considered to be an insufficient technique. However, with the advantages such as the feasibility with very low cost and the high safeness because fetal heart sound is obtained outside of a maternal body, it may be made to an effective technique that can easily monitor a fetal condition at home if the technique is provided to enable long-term monitoring even in a high-noise environment.
Meanwhile, the independent component analysis has been established as a novel method of signal analysis that can extract target signals even in a high-noise environment where signals are feebler than noises. This is a method of signal analysis to separate signals based on a difference in probability distribution of the target signals using a plurality of measured signals, and offers incomparable characteristics that it can extract the target signals while preserving overall frequency components. In actual data, however, a measured signal may often be a single signal such as in the case of the fetal phonocardiogram, so that the development of independent component analysis for a single measured signal has been expected.
As the independent component analysis for a single measured signal, a method of embedding the measured signals in a time-delay coordinate system has been proposed (see Non-Patent Document 1 below).
This is the method of converting singular value decomposition in the time-delay coordinate system (see Non-Patent Document 2 below) to the independent component analysis. Specifically, given that measured data x=(x1, x2, x3, . . . , xN), m-dimensional data as described by Formula 1 is created using the time delay from x, and the singular value decomposition or the independent component analysis is conducted therefor to extract the target signals.
                    X        =                  [                                                                                          x                    1                                    ,                                      x                    2                                    ,                                      x                    3                                    ,                  …                  ⁢                                                                          ,                                      x                    n                                                                                                                                            x                                          1                      -                      τ                                                        ,                                      x                                          2                      -                      τ                                                        ,                                      x                                          3                      -                      τ                                                        ,                  …                  ⁢                                                                          ,                                      x                                          n                      -                      τ                                                                                                                                                                x                                          1                      -                                              2                        ⁢                                                                                                  ⁢                        τ                                                                              ,                                      x                                          2                      -                                              2                        ⁢                        τ                                                                              ,                                      x                                          3                      -                                              2                        ⁢                        τ                                                                              ,                  …                  ⁢                                                                          ,                                      x                                          n                      -                                              2                        ⁢                        τ                                                                                                                                                …                                                                                                          x                                          1                      -                                                                        (                                                      m                            -                            1                                                    )                                                ⁢                        τ                                                                              ,                                      x                                          2                      -                                                                        (                                                      m                            -                            1                                                    )                                                ⁢                        τ                                                                              ,                                      x                                          3                      -                                                                        (                                                      m                            -                            1                                                    )                                                ⁢                        τ                                                                              ,                  …                  ⁢                                                                          ,                                      x                                          n                      -                                                                        (                                                      m                            -                            1                                                    )                                                ⁢                        τ                                                                                                                          ]                                    [                  Formula          ⁢                                          ⁢          1                ]            
However, as disclosed in Non-Patent Document 3 below, this method has a disadvantage that the independent component analysis involves addition/subtraction within each row and therefore the mixture of time is caused, leading to randomness in time coordinates of the obtained analysis results. Although various attempts have been made to overcome this problem, a fundamental solution has not been found.
As disclosed in Patent Document 1 below, for example, the inventors of the present invention have been studied on the independent component analysis with reference for searching target signals based on reference signals in the independent component analysis.